Supposedly you can make use of Bitcoin anonymous by using Tor. We doubt it. Here's something recently posted at Reddit:

Anything in Tor is completely traceable. This is an important point, so I'll reiterate it in big scary letters:

TOR IS COMPLETELY TRACEABLE

Any low-latency mixnet is. There's no two ways about it. Anyone who can observe a sufficiently large part of the internet over time can correlate your traffic to that of the server and get a pretty good idea that it's you.

I'll observe your IP connecting to an entry node and sending 13981 bytes to it. Then I'll observe an exit node sending exactly 13981 bytes to a server a second later. That's you, with an overwhelming probability. It's a bit of a simplification, but it's really that easy. I just have to be powerful enough to be able to observe large parts of the internet, since Tor nodes are spread around the globe.